The present invention relates generally to gas diffusion electrodes of the type used in electrochemical devices containing a solid polymer electrolyte membrane and relates more particularly to a novel gas diffusion electrode of the aforementioned type and to a method of manufacturing the same.
Electrochemical devices of the type comprising a solid polymer electrolyte membrane (PEM) are well-known, such electrochemical devices finding applications as, for example, fuel cells, electrolyzers, sensors, gas concentrators, gas compressors, supercapacitors, ultracapacitors and industrial electrochemical process units. A common type of solid polymer electrolyte membrane that is used in electrochemical devices consists of a homogeneous perfluorosulfonic acid (PFSA) polymer, said PFSA polymer being formed by the copolymerization of tetrafluoroethylene and perfluorovinylether sulfonic acid. See e.g., U.S. Pat. No. 3,282,875, inventors Connolly et al., issued Nov. 1, 1966; U.S. Pat. No. 4,470,889, inventors Ezzell et al., issued Sep. 11, 1984; U.S. Pat. No. 4,478,695, inventors Ezzell et al., issued Oct. 23, 1984; U.S. Pat. No. 6,492,431, inventor Cisar, issued Dec. 10, 2002, all of which are incorporated herein by reference. A commercial embodiment of a perfluorosulfonic acid polymer PEM is available from DuPont (Wilmington, Del.) as NAFION® PFSA polymer.
Typically, the solid polymer electrolyte membrane is sandwiched between a pair of electrodes at the membrane interfaces on which desired electrochemical reactions take place, one of the electrodes functioning as an anode and the other of the electrodes functioning as a cathode. A first catalyst layer is typically positioned between the anode and the membrane, and a second catalyst layer is typically positioned between the cathode and the membrane, the catalyst layers either being formed as part of the electrodes or being applied to the solid polymer electrolyte membrane. The combination of the membrane, the catalysts and the electrodes is commonly referred to in the art as a membrane electrode assembly (MEA).
Where the electrochemical device is used, for example, to generate electricity, a fuel, such as hydrogen, is supplied to the anode, and an oxidizing agent, such as oxygen or air, is supplied to the cathode. At the anode catalyst layer, the fuel is oxidized, thereby forming cations and free electrons. At the cathode catalyst layer, the oxidizing agent is reduced by taking up electrons. The cations formed at the anode catalyst layer migrate through the membrane to the cathode and react with the reduced oxidizing agent. In this manner, where hydrogen is used as the fuel and oxygen is used as the oxidizing agent, water is formed at the cathode.
As can be appreciated, the electrodes serve a variety of purposes including discharging the current produced at the catalysts to neighboring current collectors and allowing the reaction gases to diffuse through the electrodes to the catalysts. In addition, the cathode desirably prevents water formed during the reaction from flooding the cathodic catalyst since such flooding prevents reaction gases from reaching the cathodic catalyst. Therefore, the electrodes must be electrically conductive, must have sufficient gas diffusion capacity for the reaction gases to reach the catalysts and, at least in the case of the cathode, must have sufficient hydrophobicity, at least in regions facing the membrane, to prevent water formed during the reaction from flooding the cathodic catalyst. The electrodes also provide structural support to the membrane, particularly where the membrane is made thin (to minimize resistance thereacross) and has little inherent rigidity.
One common type of material used as a gas diffusion electrode is carbon fiber paper. (Carbon fiber cloths, perforated metal sheets, sintered metal particle sheets, and metal meshes are other types of media also commonly used as gas diffusion electrodes.) Carbon fiber paper is a random or non-woven mat of carbon fibers. Typically, carbon fiber paper is made by preparing a slurry in water of dispersed polyacrylonitrile carbon fibers and a phenolic binder; spreading the slurry out to a desired thickness; and then heating the slurry at a sufficiently high temperature for a sufficient duration to carbonize the fibers and binder. Examples of carbon fiber paper gas diffusion electrodes are disclosed in the following U.S. patents, both of which are incorporated herein by reference: U.S. Pat. No. 4,851,304, inventors Miwa et al., which issued Jul. 25, 1989; and U.S. Pat. No. 6,713,034, inventors Nakamura et al., which issued Mar. 30, 2004.
One shortcoming that the present inventors have noted in connection with the above-described gas diffusion electrodes is that there is a tendency for such electrodes, due to rigid microscopic irregularities therein, to puncture the adjacent polymer electrolyte membrane, causing electrical shorting of the electrochemical device.
Another shortcoming that the present inventors have noted in connection with the above-described gas diffusion electrodes is that there is a tendency for such electrodes, even in the case of hydrophobized carbon paper, to become flooded at the cathode catalyst when humidification is high or reactant flow is low.
Other documents of interest include the following, all of which are incorporated herein by reference: U.S. Pat. No. 6,010,606, inventors Denton et al., which issued Jan. 4, 2000; U.S. Pat. No. 6,103,077, inventors DeMarinis et al., which issued Aug. 15, 2000; and U.S. Pat. No. 6,451,470, inventors Koschany et al., which issued Sep. 17, 2002.